override unlink methods to fix cascade delete fiscal_year>periods>date_ranges

--HG-- branch : 16.0

models/account_fiscal_year.py

@@ -80,3 +80,9 @@
                 )
 
         return True
+
+    def unlink(self):
+        periods = self.period_ids
+        for period in periods:
+            period.unlink()
+        return super().unlink()